This invention relates to a superconductor, more particularly, to a compound film superconductor.
A-15 type binary (two-element) compounds such as niobium nitride (NbN) and niobium germanide (Nb.sub.3 Ge) had been known as high Tc superconductors, but the superconducting transition temperature of these materials was only about 24.degree. K at highest. Perovskite type ternary (three-element) compounds have been expected to show higher transition temperatures, and a Ba--La--Cu--O high-Tc-superconductor has been proposed (J. B. Bendorz and K. A. Muller, Zetschrift fur Physik B--Condensed Matter, 64, 189-193, 1986).
Recently it was reported that Y--Ba--Cu--O type compounds could form superconductors with even higher transition temperatures than those of prior Ones (M. K. Wu et al, Physical Review Letters, Vol. 58, No. 9, 908-910, 1987).
The superconducting mechanism of the Y--Ba--Cu--O type materials is not yet definitely known, but there is a possibility that these materials could have respectively transition temperatures higher than the temperature of liquid nitrogen and it is expected that such compounds would show more useful properties than the conventional binary compounds as high-Tc-superconductors.
More recently, as superconductive materials with higher transition temperatures, Bi--Si--Ca--Cu--O system (H. Maeda et al, Japanese Journal of Applied Physics, 27, 1988, L209) and Tl--Ba--Ca--Cu--O systems (Z. Z. Sheng et al, Nature, 332, 1988, L139) have been discovered, both of which have transition temperatures higher than 120 K.
However, with the present technical level, these oxide superconductive materials can only be produced by a sintering process, so that these superconductive materials are available only in the form of powders or blocks of ceramics. In the practical applications of these types of materials, it is strongly desired that they are made into a film or wire. It has been, however, very difficult with the conventional technique to obtain a desired film or wire from these materials because of the poor reproducibility and a lack of reliability of their superconductive properties when they are made into a film or wire.
The present inventors found that it is possible to form from these materials a high Tc superconductor film retaining well the characteristics of these materials by using film producing techniques such as sputtering etc. and adding a novel concept to the interface structure of the superconductor film. Based on this finding and by further incorporating unique ideas in the film substrate, the present inventors have succeeded in creating a novel superconducting film structure